The fuel tax is indefensible on environmental grounds, since it inevitably leads to a “I’m paying all this tax, so I’d better use the bloody thing” mindset, reducing the likliehood of people with cars using other means of transport on occaision.
If “the man” wants to reduce fuel consumption, he should tax fuel. Simple, direct, cheap and fair.
That said, “the man” likes the money, and isn’t very into logic or fairness, so we’re stuck with these taxes, in Europe and in Taiwan.
I think there must be cases where the capacity of an engine could be increased significantly, without paying extra tax, by re-boring to the maximum (and perhaps using a spacer-plate between block and head, though that’ll be detectable externally and would reduce the compression ratio.)
It might not be worth doing specially, but if you were having an engine rebuilt anyway (e.g. the recent Little Yellow Truck thread) it might be worth maximising the (undetected) capacity increase.
Are there any commonly available engines where this would be a useful option? Most likely cases would be where there are small and large capacity engines with a common block.